Global warming in recent years has developed into such a serious situation as to threaten even the survival of mankind in the future. The main cause of global warming has been believed to be atmospheric carbon dioxide (CO2) emitted from fossil fuels which have been used in large amounts as energy sources in the 20th century. Accordingly, in the near future, it may be no longer possible to continue use of fossil fuels at the current rate. At the same time, depletion of petroleum oil and natural gas, which were believed to be inexhaustible in the past, seems to become more likely due to increasing energy demand accompanied by the rapid economic growth of so-called developing countries, for example, China, India, and Brazil.
The solar energy is considered to be a natural energy source which is most stable as an alternative energy source of a fossil fuel and has much quantity. Especially, since the vast desert spreads out near the equator called Sun Belt Places in the world, the solar energy poured thereto is quite inexhaustible supply. Regarding the use of the solar energy, if only several percent of the desert which spreads in the southwestern U.S. is used for this purpose, it is thought possible to acquire energy of 7,000 GW. Moreover, if only several percent of the desert of Arabian Peninsula and North Africa is used, it is thought that all the energy that all mankind uses can be provided.
However, even though solar energy is considered as a very strong alternative energy, in terms of utilizing it in social activities, it has problems such that (1) energy density of solar energy is low and (2) storage and transfer of solar energy are difficult.
In order to solve the problem that the energy density of solar energy is low, proposed is a huge light concentrating device which can collect solar energy.
Because a light concentrating device is exposed to ultraviolet rays of solar light, heat, windstorm, sandstorm, or the like, a glass mirror having good weather resistance has been conventionally used. However, although the glass mirror is highly durable for environments, it has disadvantages such as breakage during transport and significantly high weights, leading to high plant construction costs for providing necessary strength of mounts on which the mirrors are installed.
To solve such problems, a method of replacing a glass mirror with a resinous reflection mirror is contemplated (for example, Patent Literatures 1 and 2).
However, when aluminum is used for a metal reflective layer of a resinous reflection mirror, there is a problem that the efficiency of using energy of solar light is low since the infrared reflectivity is low in an aluminum reflective layer.
Further, when silver is used for a metal reflective layer of a resinous reflection mirror, there is a problem that the reflection efficiency is lowered due to deterioration of the silver reflective layer, which is caused by easy corrosion of silver in natural environment, or a problem that a resin substrate as a lower layer of the silver reflective layer is deteriorated by ultraviolet rays since ultraviolet rays can easily pass through the silver reflective layer.
Further, for a resinous reflection mirror, a technique of preventing deterioration of a resin substrate caused by ultraviolet rays by installing an ultraviolet reflection film, which is obtained by laminating alternately a high refractive index layer and a low refractive index layer by a vapor deposition method, has been known (for example, Patent Literature 3).
However, in order to also reflect visible rays and infrared rays by a laminate having a high refractive index layer and a low refractive index layer for effective use of solar energy, it is necessary to form multiple layers of the high refractive index layer and the low refractive index layer. When the lamination number of a high refractive index layer and a low refractive index layer is increased by deposition due to such reasons, there is a problem of having high production costs.